There are various methods of soil remediation of lead contaminated soil known in the art, however, all or almost all of them exhibit significant disadvantages. For example, lead can be removed from soil in situ using a complexing agent (e.g., EDTA: ethylenediamine tetraacetic acid) as described in U.S. Pat. No. 5,316,751. Where desired, alternative biodegradable complexing agents may be employed as described in U.S. Pat. No. 6,264,720. Lead-EDTA and other lead complexes are often highly stable and form relatively quickly over a relatively wide pH range. However, where such complexes are formed in situ, great care must be taken to avoid mobilizing the solubilized lead away from the site of contamination (e.g., into an aquifer).
Alternatively, lead may be electrochemically isolated from soil in a slurry by positioning the electrodes into the slurry as described in U.S. Pat. No. 4,193,854, or lead may be isolated from soil directly by placing the electrodes into the soil as described in U.S. Pat. Nos. 5,137,608 and 5,458,747. While such electrolytic methods often significantly reduce the risk of inadvertent contamination of uncontaminated areas, various difficulties remain. Among other things, and depending on the lead concentration, soil composition, and/or conductivity of the soil, electrochemical recovery may not be economically attractive. Moreover, electrochemical lead removal may not be practicable where the contaminated area is relatively large.
In still further known methods, lead can be extracted from a lead-EDTA solution that is electrolyzed to plate lead on a cathode. However, in such configurations, EDTA is typically electrochemically degraded at the anode, which renders such systems cost-ineffective. Moreover, as the concentration of the lead-EDTA complexes decreases, low mass transfer conditions are likely to develop and consequently electrolysis would operate under current limiting conditions. Such conditions will not only render electrolysis cost-ineffective, but also lead to generation of hydrogen, which is highly undesirable. Still further such conditions typically lead to dendritic lead deposits which are less useful and are difficult to recover.
Thus, although there are numerous configurations and methods for lead recovery are known in the art, all or almost all of them suffer from one or more disadvantages. Still further, disposal of the processing fluids and removal of the residual lead and EDTA from soil is often problematic. Therefore, there is still a need to provide improved compositions and methods for lead recovery from contaminated soil.